This disclosure relates to methods for separating carbon nanotubes, specifically for separating single wall carbon nanotubes.
Single wall carbon nanotubes generally have a single carbon wall with outer diameters of greater than or equal to about 0.7 nanometers (nm). Single wall carbon nanotubes generally have various lengths and can have aspect ratios that are from about 5 to about 10,000. In general single wall carbon nanotubes exist in the form of rope-like-aggregates. These aggregates are commonly termed “ropes” and are formed as a result of Van der Waal's forces between the individual carbon nanotubes. The individual nanotubes in the ropes may slide against one another and rearrange themselves within the rope in order to minimize the free energy. Ropes generally have between 10 and 105 nanotubes. In another embodiment, the single wall carbon nanotubes exist in the form of metallic nanotubes and semi-conducting nanotubes. Metallic nanotubes are those that display electrical characteristics similar to metals, while the semi-conducting nanotubes are those, which are electrically semi-conducting.
In general, the manner in which the graphene sheet is rolled up produces nanotubes of various helical structures. These structures as well as the lattice vectors (a1 and a2) are shown in FIG. 1. As may be seen from the FIG. 1, the integer numbers (n or m) of the lattice vectors (a1 and a2, respectively) n and m are added together and the tail and head of the resulting vector are placed on top of each other in the final nanotube structure. Zigzag nanotubes have (n,0) lattice vector values, while armchair nanotubes have (n,n) lattice vector values. Zigzag and armchair nanotubes constitute the two possible achiral confirmations, all other (m,n) lattice vector values yield chiral nanotubes. The right or left helical patterns of different (n,m) chirality carbon nanotubes are referred to as “handedness” and correspond to either (n,m) or (m,n) structures.
Carbon nanotubes can be used for a wide variety of applications such as for rendering plastics electrically conducting, in semiconductors, opto-electronic and electro-optical device applications, and the like. In each of the aforementioned applications, it is generally desirable to separate carbon nanotubes from the ropes that hold them together. In addition, it is desirable to separate carbon nanotubes according to diameter, length, chirality, handedness, electrical conductivity characteristics, and the like.
Separation of single wall carbon nanotubes based on their electrical conductivity characteristics has been conducted by amine-based selective solubilization, deoxyribonucleic acid (DNA) based anionic chromatography, dielectrophoresis, electrophoresis, selective reactivity against reactive reagents, density gradient centrifugation, and by other methods. Separation of single wall carbon nanotubes based on their lengths has been mainly accomplished by size-exclusion chromatographic techniques, capillary electrophoresis, and field-flow fractionation. Separation of single wall carbon nanotubes by diameter has been demonstrated by density gradient centrifugation as well as by DNA-based anionic chromatography. Separation of single wall carbon nanotubes based on their handedness or chirality was recently demonstrated by the interaction of a chiral bi-porphyrin moiety with single wall carbon nanotubes. As will be noted, DNA and other surfactant moieties are often used to facilitate the separation of single wall carbon nanotubes from ropes or small bundles that contain each other, i.e., the effective nanotube solubilization.
In addition, DNA affords multi-level separation of nanotubes according to type (electrical conductivity characteristics), length, diameter and chirality. Such separation is afforded only for specific DNA sequences (i.e., d(GT)n oligomers), which clearly is a major hurdle in terms of commercialization and scale-up due to the prohibitive cost of DNA. Moreover, the natural difficulty to desorb these DNA oligomers from the single wall carbon nanotubes in order to clean them and further process them adds another layer of complexity to DNA-processed single wall carbon nanotubes.
It is therefore desirable to find cheaper and more efficient methods of separating carbon nanotubes from each other.